It is well known that most of the bodily states in mammals including infectious disease states, are effected by proteins. Such proteins, either acting directly or through their enzymatic functions, contribute in major proportion to many diseases in animals and man.
Classical therapeutics has generally focused upon interactions with such proteins in efforts to moderate their disease causing or disease potentiating functions. Recently, however, attempts have been made to moderate the actual production of such proteins by interactions with molecules that direct their synthesis, intracellular RNA. By interfering with the production of proteins, it has been hoped to effect therapeutic results with maximum effect and minimal side effects. One approach for inhibiting specific gene expression is the use of oligonucleotides and oligonucleotide analogs as antisense agents.
Antisense methodology is the complementary hybridization of relatively short oligonucleotides to single-stranded mRNA or single-stranded DNA such that the normal, essential functions of these intracellular nucleic acids are disrupted. Hybridization is the sequence specific hydrogen bonding of oligonucleotides to Watson-Crick base pairs of RNA or single stranded DNA. Such base pairs are said to be complementary to one another.
Naturally occurring event that lead to disruption of the nucleic acid functions are discussed by Cohen in Oligonucleotides: Antisense Inhibitors of Gene Expression, (CRC Press, Inc., Boca Raton Fla., 1989). These authors proposes two possible types of terminating events. The first, hybridization arrest, denotes a terminating event in which the oligonucleotide inhibitor binds to the target nucleic acid and thus prevents, by simple steric hindrance, the binding of essential proteins, most often ribosomes, to the nucleic acid. Methyl phosphonate oligonucleotides; P. S. Miller & P.O.P. Ts'O, Anti-Cancer Drug Design," Vol. 2, pp. 117-128 (1987); and .alpha.-anomer oligonucleotides, Cohen J. S. ed., Oligonucleotides: Antisense Inhibitors of Gene Expression, (CRC Press, Inc., Boca Raton Fla. 1989), are two of the most extensively studied antisense agents that are thought to disrupt nucleic acid function by hybridization arrest.
A second type of terminating event for antisense oligonucleotides involves enzymatic cleavage of the targeted RNA by intracellular RNase H. The oligonucleotide or oligonucleotide analog, which must be of the deoxyribo type, hybridizes with the targeted RNA and this duplex activates the RNase H enzyme to cleave the RNA strand, thus destroying the normal function of the RNA. Phosphorothioate oligonucleotides are a prominent example of an antisense agent that operates by this type of antisense terminating event.
Considerable research is being directed to the application of oligonucleotides and oligonucleotide analogs as antisense agents for therapeutic purposes. Applications of oligonucleotides as diagnostics, research reagents, and potential therapeutic agents require that the oligonucleotides or oligonucleotide analogs be synthesized in large quantities, be transported across cell membranes or taken up by cells, appropriately hybridize to targeted RNA or DNA, and subsequently terminate or disrupt nucleic acid function. These critical functions depend on the initial stability of oligonucleotides towards nuclease degradation.
A serious deficiency of naturally occurring oligonucleotides and existing oligonucleotide analogs for these purposes, particularly those for antisense therapeutics, is the enzymatic degradation of the administered oligonucleotide by a variety of ubiquitous nucleolytic enzymes, intracellularly and extracellularly located, hereinafter referred to as "nucleases". It is unlikely that unmodified oligonucleotides will e useful therapeutic agents because they are rapidly degraded by nucleases. Modification of oligonucleotides to render them resistant to nucleases is therefore greatly desired.
Modifications of oligonucleotides to enhance nuclease-resistance have heretofore generally taken place on the sugar-phosphate backbone, particularly on the phosphorus atom. Phosphorothioates, methyl phosphonates, phosphoramidates, and phosphotriesters (phosphate methylated DNA) have been reported to have various levels of resistance to nucleases. However, while the ability of an antisense oligonucleotide to bind to specific DNA or RNA with fidelity is fundamental to antisense methodology, modified phosphorus oligonucleotides, while providing various degrees of nuclease resistance, have generally suffered from inferior hybridization properties.
One reason for this inferior hybridization may be due to the prochiral nature of the phosphorus atom. The modifications on the internal phosphorus atoms of modified phosphorous oligonucleotides result in Rp and Sp stereoisomers. Since a practical synthesis of stereoregular oligonucleotides (all Rp or Sp phosphate linkages) is unknown, oligonucleotides with modified phosphorus atoms have n.sup.2 isomers with n equal to the length or the number of the bases in the oligonucleotide. Furthermore, modifications on the phosphorus atom have unnatural bulk about the phosphodiester linkage that interferes with the conformation of the sugar-phosphate backbone and consequently the stability of the duplex. The effects of phosphorus atom modifications cause inferior hybridization to the targeted nucleic acids relative to the unmodified oligonucleotide hybridizing to the same target.
The relative ability of an oligonucleotide to bind to complementary nucleic acids may be compared by determining the melting temperature of a particular hybridization complex. The melting temperature (T.sub.m), a characteristic physical property of double helixes, denotes the temperature in degrees centigrade at which 50% helical versus coil (unhybridized) forms are present. T.sub.m is measured by using the UV spectrum to determine the formation and breakdown (melting) of hybridization. Base stacking that occurs during hybridization, is accompanied by a reduction in UV absorption (hypochromicity). Consequently, a reduction in UV absorption indicates a higher T.sub.m. The higher the T.sub.m, the greater the strength of the binding of the strands. Non-Watson-Crick base pairing has a strong destabilizing effect on the T.sub.m. Consequently, absolute fidelity of base pairing is likely necessary to have optimal binding of an antisense oligonucleotide to its targeted RNA.
Considerable reduction in the hybridization properties of methyl phosphonates and phosphorothioates has been reported: see Cohen, J. S., ed. Oligonucleotides: Antisense Inhibitors of Gene Expression, (CRC Press, Inc., Boca Raton Fla., 1989). Methyl phosphonates have a further disadvantage in that the duplex it forms with RNA does not activate degradation by RNase H as an terminating event, but instead acts by hybridization arrest that can be reversed due to a helical melting activity located on the ribosome. Phosphorothioates are highly resistent to most nucleases. However, phosphorothioates typically exhibit non-antisense modes of action, particularly the inhibition of various enzyme functions due to nonspecific binding. Enzyme inhibition by sequence-specific oligonucleotides undermines the very basis of antisense chemotherapy.
Therefore, oligonucleotides modified to exhibit resistance to nucleases, to activate the RNase H terminating event, and to hybridize with appropriate strength and fidelity to its targeted RNA (or DNA) are greatly desired for anti-sense oligonucleotide diagnostics, therapeutics and research.